1. Field of the Invention
The present invention relates to the field of the electronic equipments and in particular it concerns a low phase-noise device, like e.g. a voltage-controlled oscillator mounted on microstrip, and a method of reducing such noise in such devices or other devices comprising a microstrip-mounted coaxial ceramic resonator.
2. Background Information
In a microstrip- mounted low-noise voltage-controlled oscillator (VCO) wherein a section of ceramic coaxial transmission line is used for simulating a high-Q (figure of merit) inductor, it is known that the phase noise characteristic can be seriously affected by the parasitic capacitance existing between the resonator terminal area and the ground plane. The problem is particularly evident where the tuning range of the oscillator is small i.e. when the varactor diode, being weakly fixed to the resonator, scarcely affects the phase noise. Naturally, the situation even worsen when a low-cost epoxy resin substrate with a rather high dielectric constant is used, wherein the parasitic capacitance is higher and the figure of merit Q is rather small.
Hence the technical problem to be solved in practice consists in providing a practical and effective system for reducing the unavoidable parasitic capacitance, and for assuring that its Q is as high as possible, well greater than allowed by the substrate losses so as not to excessively impair the overall figure of merit of the resonant circuit and thus the phase noise of the oscillator.
A known solution to such a problem provides for mounting the critical components in a risen fashion or "in the air". In other words, according to this known solution, the two coupling capacitors, one being connected to the active device and the other being connected with the varactor diode, are directly soldered between the terminal of the resonator and the respective lands of the printed circuit without touching the substrate surface. In this way it is clear that, disposed in series with the low-Q capacitance, represented by the substrate, is a second capacitance disposed between the resonator terminal and the upper face of the microstrip circuit which, having the air as dielectric medium, takes on a very small value with respect to the first one and is characterized by low losses. On the other hand, it is easy to verify that the combination of the above two capacitances can be represented by a single equivalent parasitic capacitance whose characteristics are close to those of the sole air capacitance; as a result, a remarkable reduction of the parasitic capacitance to ground is obtained in parallel with the resonator and the figure of merit is increased. Although this solution substantially solves the above-mentioned problem, it raises evident problems of practical and functional nature since it requires a manual positioning and soldering of the two small capacitors (operations which are obviously time-consuming, uneasy and expensive). In addition, the resonator terminal should be slightly shortened by way of hand mechanical operations which are still time consuming, inaccurate, often jeopardize irremediably the terminal itself and, last but not least, they prevent the mounting operations from being automated.